A. Field of Invention
The present invention relates to a hydrodynamic bearing, and, in particular, to a hydrodynamic bearing for rotatably supporting a rotor about a stationary shaft. The present invention also relates to an electric motor having a rotor supported on the stationary shaft through the hydrodynamic bearing.
B. Description of Related Art
FIG. 1 is a cross sectional view of a conventional spindle motor (an electric motor) provided with a hydrodynamic bearing which makes use of the dynamic pressure of a lubricating fluid for driving at least one data storage media such as hard disk.
Referring to FIG. 1 the spindle motor includes a rotational sleeve d which constitutes a part of a rotor rotatably supported through a lubricant fluid e by a stationary shaft c having a thrust plate b radially outwardly extending from a shaft member a through a lubricating fluid e.
An axially lower end portion (base portion) of the stationary shaft c is fixed to base member f and an axially upper end portion (top portion) is fixed to, for instance, a cover member of a hard disk drive. The rotational sleeve d includes a journal portion di and a grooved portion g defined within the inner periphery surface of the journal portion d1. The inner peripheral surface of the journal portion d1 and the outer peripheral surface of the shaft member oppose with each other through a gap beneath the grooved portion g toward the axially lower end portion of the rotational sleeve d. And the thrust plate b is positioned within the grooved portion g though a gap.
The lubricating fluid is filled within each gap thereby a pair of radial bearings i are constituted between the inner peripheral surface of the journal portion d1 and the outer peripheral surface of the shaft member a and a thrust bearing is also constituted between the upper and lower surface of the grooved portion g and the upper and lower surface of the thrust plate b.
In the spindle motor having the conventional hydrodynamic bearing device, the lubricating fluid e of the radial bearing i sometimes moves toward the axially upper side (toward the top portion of the stationary shaft c) due to various factors such as production tolerances, errors during assembly or heat deformation during rotation of the rotor d. The movement of the lubricating fluid e as mentioned above leads to reduction in the amount of the lubricating fluid held in the radial bearing i. Such reduction of the lubricating fluid e causes problems such as reduction of the precision of the rotation and the operation life of the hydrodynamic bearing device or even the spindle motor.
On the other hand, there are other conventional hydrodynamic bearing devices that include an annular space filled with air interposed to separate the lubricating fluid held in the radial bearing and the lubricating fluid held in the thrust bearing. In the case of such hydrodynamic bearing device, since the lubricating fluid held in the radial bearing i moves toward the axially lower side (toward the base portion of the stationary shaft c) due to various factors such as production tolerances, errors during assembly or heat deformation during rotation of the rotor d. The movement of the lubricating fluid e as mentioned above leads to reduction of the amount of the lubricating fluid held in the radial bearing i. Such reduction of the lubricating fluid e causes similar problems as above described.
In the conventional hydrodynamic bearing device shown in FIG. 1, when the rotor d does not rotate, the lubricating fluid is held in the gap so as to retain the lower end side interface e1 of the lubricating fluid e within a tapered portion h by surface tension of the lubricant. The tapered portion h is inwardly tapered toward the lower end side (base portion side) of the shaft portion a so as to form a taper seal portion.
In the conventional hydrodynamic bearing device shown in FIG. 1, when the rotor d starts to rotate, centrifugal forces are applied to the lubricating fluid e held within the gap defined between the stationary shaft c and the rotor d. As a result, the lubricating fluid e easily oozes along the inner peripheral surface of the journal portion d1 toward the axially lower side thereby causing the leakage of the lubricating fluid e.